Plasma activated water as sustainable method to control bacterial disease severity on tomato plants

The bacterial plant disease control is nowadays limited to copper compounds, few resistance inducers and biocontrol agents, while prophylaxis still remains the most important management tool. In this study, the efficacy of plasma activated water (PAW) as innovative alternative in the control of plant diseases caused by bacterial pathogens is investigated. To assess the PAW efficacy as resistance inducer, tomato plants experimentally inoculated with Xanthomonas vesicatoria (Xv) were used as model. 80 ml of sterile distilled water (SDW) have been treated with plasma by means of a nanosecond pulsed dielectric barrier discharge (DBD) source, operating at atmospheric pressure and ambient air [1]. The electrodes of the plasma source consist of aluminum foils, covered by 1 mm thick polystyrene as dielectric; one of the polystyrene dielectrics is shaped with edges in order to contain the water to be treated. The plasma forms in the 1 mm gap region between the liquid surface and the upper dielectric layer. Ten minutes treatment time with a peak voltage of 19 kV and a pulse repetition frequency of 1,000 Hz generates in plasma treated water nitrates, nitrites and peroxides and induces a reduction of pH. Plants of 'Moneymaker' and 'VF-10' tomato cultivars, were grown in pots under greenhouse condition until they reached the 3rd – 4th leaf stage. The roots of tomato plants were then soaked for 10 min in PAW, SDW (positive control) and in a solution of acibenzolar-S methyl (a resistance inducer, as negative control) six-days before the inoculation with the pathogen. Experimental inoculation with Xv strain IPV-BO 2684 was carried out by spraying the pathogen aqueous suspension (ca. 1×107 CFU/mL) on tomato leaves. The phytopathometric assessments were carried out 21 days after pathogen inoculation by counting the number of spots on tomato leaves. The plants pretreated with PAW showed a significantly low number of leaf spots with respect to those pretreated with SDW. The relative protection of PAW related to negative control was estimated in the range 30%-40%, according with the tomato cultivar tested. No phytotoxicity effect has been observed. These results highlighted that the preventive application of PAW at the root apparatus was able to reduce the disease severity (number of leaf spots) implying the activation of plant defence responses